Field of the Invention
The instant invention relates to a trimming resistor, especially a miniature trimming resistor, comprising a housing, a resistance plate including a brush track, a brush resiliently biased against the brush track, and a rotary driver for the brush, being supported in the housing. Such trimming resistors are commercially available in the most varied structural forms. It is customary in general to design the brush or wiper contacts as hemispherical projections of spring arms or brush or wiper arms. These arms consequently serve a dual purpose: They establish an electrical connection including the contact making and they provide the contact pressure force.
An earlier brush spring of a trimming resistor devised by the applicant of the instant application comprises a flat body from which two lateral arms originate that are bent towards each other inwardly by a bending angle in the order of 170.degree.. The ends of both arms each carry a brush contact. One of the arms is recessed in the middle so that the other arm may pass through this recess.
At extreme miniaturization great expenditure is required if such a wiper spring is to be produced accurately. Furthermore, the exact location of the meeting point of the contact and a "brush track" as well as the mutual spacing of the two contacts depends on the bending angle of the two arms. At extreme miniaturization this calls for strict tolerance requirements to be met and results in correspondingly great numbers of rejects. Finally, at extreme miniaturization the spring pressure or contact pressure required for sufficient contact making between the contact and the wiper track can no longer be obtained with the usual resilient contact materials.
It is, therefore, an object of the instant invention to improve the trimming resistor of the kind specified initially such that it will be easy to manufacture. It is another object of the invention to provide a trimming resistor of the kind in question which will operate perfectly during a long service life.
This object is met, in accordance with the invention, in that the brush is a substantially planar, small contact plate arranged parallel to the surface of the resistance plate, and that a separate spring ring is provided whose resilient tongue presses the small contact plate against the resistance plate.
Advantageous modifications and further developments of the invention may be gathered from the subclaims.
Briefly, the invention provides for two different structural members to fulfill the functions of "contact making" and "contact pressure generation", namely a small contact plate and a spring ring cooperating with each other by virtue of their relative positions of installation. In accordance with a further development of the invention these two structural members are so designed that they can be produced easily by stamping or punching tools, observing precise dimensions, and can be assembled and mounted with ease. Manufacturing and assembly tolerances are admissible within rather wide limits as compared to the wiper spring mentioned above, and yet that does not impair the electrical operating features of the trimming resistor.